Breaking out of a fixed rate mortgage will result in penalties, which depending on the interest the bank loses out on for the remaining term, can be in the thousands.
You could enquire with First Active as to whether you can extend the interest only repayment up until the fixed rate period ends. Hopefully by then, interest rates will still be as low as they are now, and you'll benefit from the savings.
However, always bear in mind that whilst you stay on interest-only repayments, you do no chip away at any of the capital balance of the loan.